This invention relates to circuits for use in radio communications. In particular, it relates to frequency synthesiser circuits which incorporate a VCO (voltage controlled oscillator) circuit and VCO circuits for use therein. Such circuits are useful in radio transmitters and receivers for generating stable frequency signals.
Carrier frequency signals in radio communications transmitters are conventionally generated by a frequency sythesiser circuit. Such a circuit usually includes a VCO connected in a phase locked loop (PLL). The loop, including the VCO, provides an appropriate stable output at a precisely defined frequency. Such a circuit may also be employed to generate a local osillator signal for use in a radio receiver. In many cases the transmitter and receiver are combined in a single transceiver unit.
Multimode transceiver units for use in multiple communications modes or systems having different operating properties have recently become available. These units generally include dedicated synthesiser circuits as well as dedicated transceiver circuits for the separate modes. Unfortunately, the provision of a plurality of such synthesiser circuits to cater for different operating modes in a single transceiver unit considerably increases the size and volume occupied by the circuits as well as their cost of manufacture.
The purpose of the present invention is to provide a frequency synthesiser circuit which facilitates use in a multimode radio transceiver (or transmitter or receiver) in a manner in which these disadvantages are reduced.
U.S. Pat. No. 5,537,448 describes a known VCO circuit arrangement. FIG. 1 shows a PLL wherein between a phase detector and the VCO there are a plurality of switchable loop filters (together with low-pass filters). These are provided for dynamically changing loop bandwidth whilst maintaining the same stability parameters (phase and gain margin). This arrangement does not provide the benefits obtainable with the present invention to be described herein.
According to the present invention in a first aspect there is provided a VCO (voltage controlled oscillator) circuit for connection in a phase locked loop arrangement wherein the VCO circuit has a plurality of operational states in each of which the VCO circuit is operable to provide activation of a selected one of a plurality of different phase locked loops when connected to the VCO circuit, the VCO circuit including switching means for switching the state of the VCO circuit to allow the operational state of the VCO to be selected.
According to the present invention in a second aspect there is provided a frequency synthesiser circuit for use in radio communications to generate a stable frequency signal, the circuit comprising a VCO (voltage controlled oscillator) circuit for use in a phase locked loop arrangement, the synthesiser circuit being characterised in that it comprises a plurality of different phase locked loops each having a first operable state in which the loop is activated and a second operable state in which the loop is deactivated and a VCO circuit at least part of which is connected in and shared by the loops the VCO circuit having a plurality of operational states in each of which the VCO circuit is operable to provide activation of a selected one of the different phase locked loops, the VCO circuit including switching means for switching the state of the VCO circuit to allow the operational state of the VCO to be selected.
In this specification, xe2x80x98activatexe2x80x99 or xe2x80x98activatedxe2x80x99 indicates that a phase locked loop including a VCO is operable to produce r.f. signals of a desired output frequency and xe2x80x98deactivatexe2x80x99 or xe2x80x98deactivatedxe2x80x99 indicates that it is not.
VCO devices generally comprise a tuning and resonator portion and an active portion, examples of which are described in more detail later. Where part of the VCO is shared by the phase locked loops in the circuit according to the present invention, the shared part may comprise the active portion of the VCO circuit and preferably also the resonator part of the tuning and resonator portion of the VCO circuit.
The switching means may include a plurality of electrical switches each connected to an associated part of the VCO circuit, whereby the operational state of the VCO circuit and the state of each associated phase locked loop may be selected, each loop being operably activated or operably deactivated by closing of its associated switch in the VCO circuit. Each of the switches may be connected between a voltage source and a corresponding voltage controlled device forming part of the VCO circuit. Each of the voltage controlled devices may comprise a voltage variable capacitance, e.g. a varactor device. Each voltage controlled device may form part of the VCO circuit not shared with the other loops, especially a tuning part of the VCO. Each voltage controlled device may be connected in one of a plurality of control loops, each of the control loops being associated with one of the phased locked loops. The shared part of the VCO may thereby be incorporated in each of the control loops. In such a circuit, each voltage controlled device may control whether or not its associated phase locked loop is operably activated or not by the voltage applied to the voltage controlled device, thereby determining whether it makes a significant contribution to the frequency of oscillations of the VCO (activated) or not (deactivated).
Each of the control loops to which the VCO circuit is connected may include a grounded portion and a non-grounded portion and at least part of the VCO circuit may comprise a common link in each of the control loops between the grounded and non-grounded portions of the control loops. The common link may also comprise the part of the VCO which is shared between the phase locked loops.
The phase locked loops may share a common input terminal to and a common output terminal from the shared part of the VCO. Each of the phase locked loops may include, in a portion of the loop not shared with the other phase locked loops, a capacitor. The capacitors of the different phase locked loops may be connected to the common input terminal to the shared part of the VCO. Where the circuit includes control loops as described earlier, the voltage controlled device of each of the control loops may conveniently be connected through one of the capacitors to the common input terminal of the shared part of the VCO.
Thus, the invention allows two or more PLLs to be connected to and operably associated with the same VCO. Those PLLs could have very different specifications (e.g. loop bandwidth, lock time, noise level requirements, etc) and their phase detectors could be quite different. This is not possible with the VCO described in U.S. Pat. No. 5,537,448 where the switching which occurs is external to the VCO and is straightforward switching between loop filters.
The first and second operational modes obtainable in the frequency synthesiser circuit according to the second aspect of the invention may be modes which allow communication in two different systems, e.g. in a GSM (Global System for Telecommunications) system and a TETRA (Terrestrial Trunked Radio) system. These are different digital systems which are respectively well established and recently emerging systems defined according to known international standards. Preferably, the frequency to be generated in the two systems is not excessively different. For example, it is preferred that the frequency difference between the outputs required in the different systems is not greater than 10 percent, desirably not greater than 5 percent of the mean of the frequencies to be generated.
The present invention beneficially allows a multi-mode radio transceiver unit to be built which is cheaper and occupies less space and has less components than known multi-mode units.
Embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: